The present disclosure relates to steam generating systems which can be used in combination with carbon capture sequestration (CCS) technology for use in coal-fired power generation.
During combustion, the chemical energy in a fuel is converted to thermal heat inside the furnace of a boiler. The thermal heat is captured through heat-absorbing surfaces in the boiler to produce steam. The fuels used in the furnace include a wide range of solid, liquid, and gaseous substances, including coal, natural gas, and diesel oil. Combustion transforms the fuel into a large number of chemical compounds. Water and carbon dioxide (CO2) are the products of complete combustion. Incomplete combustion reactions may result in undesirable byproducts that can include particulates (e.g. fly ash, slag), acid gases such as SOx or NOx, metals such as mercury or arsenic, carbon monoxide (CO), and hydrocarbons (HC).
FIG. 1 illustrates the steam-water flow system and the gas flow system for a conventional once-through two pass Carolina-style boiler 10 with a furnace 20 capable of operating at subcritical to supercritical pressure. As is known, the boiler 10 includes fluid cooled membrane tube enclosure walls typically made up of water/steam conveying tubes 30 separated from one another by a steel membrane (not visible) to achieve a gas-tight enclosure. The tubes 30 are referred to herein as water tubes for brevity and simplicity.
The steam generator operates with a variable pressure profile versus load (subcritical to supercritical pressure). The water enters the economizer through inlet 141 and absorbs heat, then travels from economizer outlet 142 to inlet 143 at the base of the furnace. A lower bottle (not shown) may be present to distribute this water. The water then travels up through the furnace wall tubes 30. As the water travels through these water tubes 30, the water cools the tubes exposed to high-temperature flue gas in the combustion chamber 60 and absorbs energy from the flue gas to become a steam-water mixture at subcritical pressure (and remains a single phase fluid if at supercritical pressure conditions). The fluid is discharged into the vertical steam separators 42, where the steam-water mixture is separated, when subcritical, into wet steam (i.e., saturated steam) and water. Any water can exit via downcomer 50 and pass from outlet 144 to the economizer inlet 141. When the fluid is supercritical, the vertical separators act as conveying pipes with all the entering steam leaving from the top outlets. The steam is used to cool the flue gas in the convection pass path 70 of the furnace through steam tubes or roof tubes 75 leading from the vertical separator. The steam then passes from outlet 149 to inlet 145 and is fed through superheater heating surface 80, then sent to the high pressure steam turbine (reference number 146). Steam returning from the high pressure steam turbine (reference number 147) passes through the reheater heating surface 90 to absorbing additional energy from the flue gas, and can then be sent to a second intermediate-pressure or low-pressure steam turbine (reference number 148). The steam sent to the turbines is generally dry steam (100% steam, no water). The steam from the superheater 80 heating surfaces can be sent to a high pressure (HP) turbine, then from the reheater 90 heating surface to the intermediate pressure (IP) and low pressure (LP) steam turbine stages (not shown). Feedwater conveyed through economizer 100 may also be used to absorb energy from the flue gas before the flue gas exits the boiler; the heated feedwater is then sent to the furnace enclosure tubes 30, or can be sent through superheater 80 and reheater 90.
Referring to the gas flow system, air for combustion can be supplied to the furnace 20 through several means. Typically, a fan 102 supplies air 104 to a regenerative air heater 106. The heated air is then sent as secondary air 108 to windboxes for distribution to individual burners and as primary air 110 to the coal pulverizer 112, where coal is dried and pulverized. The primary air (now carrying coal particles) 116 is then sent to the burners 120 and mixed with the secondary air 108 for combustion and formation of flue gas 130 in the combustion chamber 60. The flue gas flows upwardly through the furnace combustion chamber 60 and then follows the convection pass path 70 to flue gas exhaust 160 past superheater 80, reheater 90, and economizer 100. The flue gas can then be passed through the regenerative air heater 106 (to heat the incoming air 104) and pollution control equipment 114 and, if desired, recycled through the furnace 20. The flue gas exits the boiler 10 through the flue gas exhaust 160.
FIG. 1 also illustrates the start up equipment of the steam-water flow system. When the steam is supercritical, a vertical steam separator 42 is used instead of a conventional horizontal steam drum of a subcritical natural circulation boiler. A boiler circulation pump 44 and shutoff valve 46 are also present in the downcomer 50 to augment the flow in the furnace enclosure wall water tubes 30 and the economizer 100 during startup. The boiler circulation pump is stopped at the load where 100% dry steam is entering the vertical steam separator from the furnace enclosure. The vertical steam separator remains in service and a static column of water remains in the downcomer 50.
As illustrated here, the steam outlet terminals of a Carolina style boiler are located at the top of the boiler, generally at a relatively high elevation from grade of about 200 feet. The steam is then carried to a steam turbine via steam leads (i.e. pipes). The steam leads are made from a nickel alloy for 700° C. steam temperatures, which is very expensive. Due to the location of the steam outlet terminals at the top of the boiler, the length of the steam leads can be very great. It would be desirable to be able to reduce the length of the steam leads from the steam outlet terminals of the boiler to the steam turbine where the steam is used to generate electricity.